The Research On Pilot Allocation And Channel Estimation In Massive MIMO System Under Pilot Contamination

The explosion of traffic data volume and the extreme rise in number of wireless links make the contradiction between the limited wireless spectrum resource and the demands of people for wireless communication service become more prominent.On this background,Massive Multiple-In Multiple-Out(Massive MIMO)technology has received extensive attention once it has been proposed.Although the scenarios and requirements of the fifth generation(5G)mobile communication have been basically clear,massive MIMO technology,which is one of the candidate technologies,is also evolving.But the pilot contamination that has plagued large-scale MIMO systems has been received high attention.The thesis mainly studies the interference problem of pilot signals in Massive MIMO systems.Specific research has been done to mitigate the effects of pilot contamination to improve spectral efficiency.The thesis introduces the development history of Massive MIMO technology and the research of pilot contamination.The basic theoretical knowledge of the system model,channel estimation methods,uplink pilot and downlink data transmission is elaborated.The common schemes and algorithms of the pilot decontamination are summarized.The pilot contamination problem and its influence are analyzed emphatically,which pave the way for the future research.The thesis researches the pilot contamination problem from three aspects: pilot schedule and fractional pilot reuse,channel estimation and asynchronous fractional pilot schedule.For pilot scheduling and fractional pilot reuse,the idea of pilot scheduling is studied.The common model of fractional pilot reuse is analyzed.The capacity of the central user and the edge user are derived,and the right number of pilots are find to maximize the sum rate.The simulation also verifies that fractional pilot reuse has certain advantage in sum rate over integer reuse.At first,the whole process of uplink pilot transmission is derived.Then,the transmission process of pilot signals under MMSE receiver is analyzed.Finally,the NMSE and complexity of several common signal estimators are compared.The asynchronous fractional pilot schedule is an extension of the model of fractional pilot reuse.Users are grouped according to the amount of pilot contamination received.The users suffered from mild pilot contamination in adjacent cells all user same orthogonal pilots,while users suffered from sever pilot contamination in adjacent cells all use orthogonal pilots.Reducing inter-cell interference by using asynchronous transmission for reuse the same orthogonal pilots.The simulation resultsprove that uplink sum rate is improved significantly.